US health officials say they are investigating 109 cases of severe hepatitis of unknown origin in children, including five reported deaths, updating a country-wide alert issued in April for doctors to be on the look-out for such cases of the liver disease.

The cases have been identified over the past seven months in 25 states and territories, Dr Jay Butler, deputy director for infectious diseases at the US Centers for Disease Control and Prevention (CDC) said during a conference call on Friday.

He said about half of the 109 children diagnosed with hepatitis were also infected with a type of adenovirus, a virus that causes the common cold, but the agency is still investigating the exact cause of the illness.

The update follows investigations in the United States and several European countries of clusters of hepatitis in young children.

The CDC said it is working with counterparts in Europe to understand the cause of the infections that can cause liver damage and lead to liver failure.

This week, the World Health Organisation officials said they had reports of almost 300 probable cases in 20 countries.

In the US, 94 per cent of the children were hospitalised and eight received liver transplants.

"It's still a very rare occurrence," Butler said.

"A majority of these cases have recovered and recovered fully."

The mystery goes back to November, when Alabama health officials began looking into the first of nine cases of severe hepatitis in children in that state.

None tested positive for the viruses that commonly cause hepatitis.

However, testing was positive for adenovirus.

Butler said none of the Alabama children were vaccinated against COVID-19.

That has been ruled out as a possible cause "and we hope this information helps clarify some of the speculation circulating online".

Symptoms of hepatitis, or inflammation of the liver, include fever, fatigue, loss of appetite, nausea, vomiting, abdominal pain, dark urine, light-coloured stools, joint pain and jaundice.